NOTE: Transfectants were grown for 3 days in the presence of the indicated drug; EC50 values were determined as described in Materials and Methods. Values are the mean ± SE of 2 to 34 independent experiments (n) determined in duplicate or triplicate. The resistance factor was calculated as the ratio of the EC50 value of the indicated HEK-MRP5 transfectant to the HEK-vector transfectant.